1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a photoreactive compound which can be aligned by photoreaction and includes fluorene in its backbone to have superior alignment controlling ability, a liquid crystal alignment layer using the compound, a method of manufacturing the alignment layer, and a liquid crystal display device including the alignment layer.
2. Description of the Related Art
Recently, liquid crystal displays (LCDs) have come to prominence as replacements for cathode ray tubes due to their advantages of light weight and low power consumption. In particular, a thin film transistor liquid crystal display (TFT-LCD) driven by a TFT has high response speed of liquid crystal since individual pixels are independently driven, which makes it possible to realize high quality moving images. Thus, TFT-LCDs are widely used in notebook computers, wall tapestry type TVs, etc.
When a general color TFT-LCD is manufactured, a TFT driving device and an ITO (indium titanium oxide) transparent electrode are deposited on a glass substrate, and then an alignment layer is deposited thereon to form a lower substrate of a cell. A space for injecting a liquid crystal material is formed between a pair of upper and lower substrates using a sealant. A polarization film is coated on an outer surface of the glass substrate. Finally, a liquid crystal material is injected into the space between the pair of substrates and cured to complete a LCD cell.
In order to use liquid crystal as an optical switch in a TFT-LCD, the liquid crystal should be initially aligned in a certain direction on a TFT layer which is the innermost layer of a display cell. To this end, an alignment layer is required.
It is known that an alignment layer is manufactured by rubbing a polymer layer, such as a polyimide resin layer, formed on a substrate with a cloth etc. in one direction or by obliquely depositing silicon dioxide (SiO2) on a substrate. When an alignment layer is manufactured using the rubbing process, contamination may be caused due to contact with impurities during rubbing, a product yield may be reduced due to occurrence of static electricity, and contrast may be reduced. When oblique deposition is used, the production costs increase and it is difficult to form a large area, and thus it is not suitable for a large LCD.
To solve the above problems, an alignment method comprising a non-rubbing process using a photopolymerizible alignment material was developed in which photopolymerization is caused by light irradiation to induce an arrangement of polymers, thereby aligning liquid crystal. An example of such a non-rubbing process is photo-alignment by photopolymerization, which is reported by M. Schadt et al. (Jpn. J. Appl. Phys., Vol 31, 1992, 2155), Dae S. Kang et al. (U.S. Pat. No. 5,464,669), and Yuriy Reznikov (Jpn. J. Appl. Phys. Vol. 34, 1995, L1000). Photo-alignment refers to a mechanism in which a backbone of a polymer is arranged in a certain direction, thereby aligning liquid crystal, when a photosensitive group bonded to the polymer undergoes a photoreaction due to pre-polarized ultra violet rays.
Japanese Patent Laid-Open Publication No. Hei 11-181127 (6 Jul. 1999) discloses a method of manufacturing a polymeric alignment layer, in which a backbone is acrylate, methacrylate, etc. and a side chain includes a photosensitive group, such as a cinnamic acid group, and an alignment layer manufactured by the method. However, such an alignment layer has lower anchoring energy than an alignment layer manufactured using a rubbing process, and a twist angle is about 88°, resulting in low contrast, which makes it difficult to obtain high-definition picture quality.
Further, Korean Patent Laid-Open Publication No.1998-78124 (16 Nov. 1998) discloses a cinnamate-based photosensitive polymer. Likewise, contrast ratio is still low.